Luis is 3 times as old as Daniel and is also 20 years older than Daniel. How old is Daniel?
Explanation: We can use the given information to write down two equations that describe the ages of Luis and Daniel. Let Luis's current age be $l$ and Daniel's current age be $d$ $l = 3d$ $l = d + 20$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $d$ , and both of our equations have $l$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3d$ $-$ $ (d + 20)$ which combines the information about $d$ from both of our original equations. Solving for $d$ , we get: $2 d = 20$ $d = 10$.